1. Field of the Invention
This invention relates to a method of recovering hydrocarbons from hydrocarbon-containing subterranean formations, and more particularly to methods for enhancing the recovery of both oil and natural gas from formations having a natural gas-containing zone overlying a liquid petroleum-containing zone.
2. Description of the Prior Art
Primary oil recovery is usually achieved by penetrating an oil-bearing earth formation with a plurality of wells and recovering the oil from these wells by means of the natural fluid pressure in the subterranean earth formation. In order to recover additional oil, various secondary and/or tertiary enhanced oil recovery techniques have been proposed and some of these techniques have been used commercially. For example, water flooding has been somewhat successful in recovering secondary oil from oil-bearing formations. One of the more promising enhanced oil recovery techniques involves the injection of an oil-miscible fluid, such as carbon dioxide (CO.sub.2), to increase oil recovery. In this process, the oil-miscible fluid is pumped into the formation under sufficient pressure so that it becomes miscible with oil, typically swelling the oil, decreasing its viscosity and lowering its interfacial tension. This swelling and reduction of the viscosity and interfacial tension of the oil tends to increase its mobility so as to facilitate its recovery from the oil-bearing formation.
Various methods have also been proposed to enhance the recovery of natural gas from natural gas-containing subterranean formations, such as by the displacement of the natural gas by a less valuable gas, such as nitrogen.
Some subterranean formations have an upper zone containing natural gas (or "gas cap") overlying a liquid-petroleum-bearing zone. A partially depleted liquid petroleum-containing formation may contain a significant amount of liquid petroleum (or "undrained oil") in the gas cap. This region of coexisting gas and undrained oil can result when either a primary gas cap has been allowed to expand in size or a secondary gas cap has been created by gas injection to encompass what had formerly been a liquid petroleum-containing zone. In view of the fluctuating relative values of natural gas and liquid petroleum, it has been the practice to recover the more valuable fluid from such reservoirs, often at the expense of reducing the ultimate recovery of the other fluid. For example, natural gas may be rapidly produced with the result that a natural water drive may invade the liquid petroleum-containing zone and effect a reduction in the amount of oil ultimately recoverable from that zone. Thus, a need exists for a method to enhance the recovery of both natural gas and liquid petroleum from such formations.
It is therefore an object of this invention to provide a process for recovering both natural gas and liquid petroleum from a subterranean hydrocarbon-containing formation.
It is a further object of this invention to provide a method for recovery of both natural gas and liquid petroleum from such formations while minimizing the expense of such recovery by utilizing inexpensive displacement fluids.
Another object of this invention is to provide a method for recovery of both natural gas and liquid petroleum which makes maximum advantage of any fluid density differences between the injected and connate fluids in order to minimize the adverse-gravity driven dispersion which might otherwise reduce the rate of hydrocarbon recovery.
Further objects, advantages and features of the invention will become apparent to those skilled in the art from the following description when taken in conjunction with the accompanying drawing.